In the average movie theater, two types of "surround" systems are used-the 70 mm 6-track magnetic system, and the more common 35 mm optical arrangement. The former uses a magnetic strip attached to the film to supply six discrete channels, and the latter uses two optical audio tracks. This two-channel system is the basis for home surround sound decoders.
Every stereo videodisc, tape and MTS broadcast that was surround encoded still contains the same rear channel information as the two-channel magnetic master from which the theatrical 35 mm optical soundtrack was produced. In other words, your stereo videotape or disc of Star Trek I, II, II, Raiders of the Lost Ark, Superman and Star Wars can be decoded to produce surround sound at home. In addition, LPs, CDs and any stereo audio material can benefit from surround sound decoding. Ambiance extraction is a pleasant side effect that many decoders provide. In a nutshell, if the recording was made in a large hall, or a small club, "surround sound" will reproduce the recording environment faithfully.
Assuming the listener is seated centered between the two speakers, sound which is recorded "in phase" and with equal amplitude in each channel in a standard stereo system will appear to the listener to be located equidistant between the two speakers, as the two in-phase audio signals add together. The sound can be shifted left-to-right by varying the ratio of the amplitude of the left and right signals.
"Out of phase" signals, on the other hand, tend to cancel each other out. If a signal is recorded at equal amplitude on each channel of the stereo but 180.degree.out of phase, the listener would ideally hear nothing, as the two signals cancel each other out. As a practical matter, the signals are audible, but sound odd.
By subtracting the left and right signals (L-R), the in-phase signals will be cancelled, and the out-of-phase signals are recovered. This is the basis of the "matrix encoding" which is used to record surround information which is inaudible to listeners with conventional stereo equipment.
"Dolby Surround", a proprietary technique of Dolby Laboratories, inc., is the current standard for multi-channel movie sound. The Hollywood mixers start with a conventional stereo soundtrack, which has one left channel and one right. By using some of Mr. Dolby's black boxes, they drop in two more "matrix"-encoded channels--one for the front center channel (used mainly for dialogue), and one for the rear surround channel (used mainly for effects). The rear-channel sound information is mixed "out-of-phase" into both stereo channels ("left-minus-right"), and the center-channel information is derived from the information common to both stereo channels ("left-plus-right").
The center and surround channels must then be decoded from the encoded stereo signal. The center and rear (surround) signals are then reproduced on speakers located between the normal front stereo speakers and behind the listener, respectively.
There are many surround sound decoders on the market today. The simplest of them is the Dynaco model QD-1, which is a version of the decoder described in a 1970 Audio Magazine article by David Hafler for use with the then-emerging quadrophonic sound technology (which has since been abandoned). Hafler's U.S. Pat. No. 3,697,692 is essentially the same as the Dynaco QD-1. The Hafler system operates at high levels - that is, the speaker output from the left and right amplifiers is divided among the four speakers, with the (L+R) center speaker connected between the "-" terminal of the L and R speaker and ground, and the (L-R) rear speaker connected across the "+" terminals of the L and R speakers.
Ranga, U.S. Pat. No. 4,132,859, is another high-level system, which is a further development of the Hafler system.
Very good results can be obtained with the Hafler system. However, all high-level systems have a number of basic problems, not the least of which being the expense of using high-power components (L-Pads) to balance the system. Also, the balance controls on the amplifier must be carefully set, using a mono signal, for minimum surround channel output, and then left strictly alone. Any change in the amplifier balance destroys the surround effect.
Most surround decoders currently on the market operate at "line level". That is, they take the left and right signals at preamp level, before they are fed into the final amplifiers. This requires a second set of amplifiers for the two derived channels, but eliminates the need to deal with the power requirements of a high-level decoder. Since the surround channel signals are decoded at constant preamp level, the balance controls on the amplifier (after the decoding) have no effect on the decoding.
All of the low-level decoders known to the inventor use active components (transistors, operational amplifiers, etc.) to decode the surround information from the stereo source. The original decoders were primarily analog circuits, such as may be seen in Holbrook, U.S. Pat. No. 4,612,663, Ito, et.al. (Sansui), U.S. Pat. No. 3,757,047, or Iida (Sony), U.S. Pat. No. 3,725,586. Other low-level active analog systems are Ohta, et. al. (Victor of Japan), U.S. Pat. No. 3,745,254 (using frequency-dependent phasing), Ito, et. al, (Sansui) U.S. Pat. No. 3,761,631 (phase modulates rear channels at an ultra-low frequency rate).
More modern higher-end units today tend to use digital signal processing to achieve the same results. Various kinds of filtering, noise reduction, reverberation, and other effects are often built into these units. All of this adds to the expense and complexity of the decoders. For example, the SONY TAE-1000ESD Surround-sound Processor/pre-amp lists for approximately $1000, and offers a wealth of digital-processing modes, including one of the finest overall surround-sound decoders available; the LEXICON CP-1 Surround-sound Decoder lists for $1250, and has true Dolby Pro-Logic Surround circuitry, 16-bit digital delay, two audio/video inputs, and a full-function wireless remote control. The CP-1 also features an "auto azimuth correction" mode designed specifically to prevent dialogue from leaking into the rear channel, and a number of digital signal processing effects modes.
All of these active decoding systems, especially the digital ones, involve complicated and expensive electronics, and relatively high prices.
The Dolby Surround System introduces a digital delay into the surround (rear) channel. There are several reasons advanced for this. One is to delay the rear signal so that the front and rear signals arrive at the listener's ears at the same time. This would appear to be a poor technique, since it would depend entirely on where the listener sits relative to the two sets of speaker. Others suggest that the "Haas effect" causes a listener to localize sound to the direction it is heard first. By delaying the rear sound by a fixed amount, usually 20 milliseconds, the listener is tricked into hearing the sounds as being primarily front/center, and the effect of stray sounds being erroneously shifted to the rear is minimized. Some units add a variable delay control, which allows the user to change the length of the fixed delay, but whatever the user chooses, the delay remains fixed at whatever the chosen length is.
Twenty milliseconds is the period of one cycle at a frequency of 50 Hz. This means that the only sounds which are correctly phased with a 20 ms delay system are those which are even multiples (harmonics) of 50 Hz. All others are to a greater or lesser degree out of phase. Frequencies between the peaks can be greatly attenuated or cancelled completely due to out-of-phase mixing. This creates a situation which is every audio engineer's nightmare--an overall system response with a peak in every octave, caused by speakers which are in phase only near certain frequencies. It is advantageous, then, to eliminate the use of delays in the surround sound decoding.